Return `Result<&str, Utf8Error>` instead of `Option<&str>`

[nasso]

I don't understand why many methods return Option<&str> instead of Result<&str, Utf8Error> when decoding a string.

It hides information that could be used by the caller to form a more meaningful error message, and doesn't prevent anyone from just calling unwrap, expect or ok when they don't care about the error.

Furthermore, it would disambiguate the return value of some methods, such as Reference::symbolic_target, for which None may be returned if the reference is either not symbolic or not a valid utf-8 string. If that method returned Result<Option<&str>, Utf8Error> (or Option<Result<&str, Utf8Error>>, though one can switch between the two with (Option|Result)::transpose), it would make it easier to disambiguate with a single function call.

I couldn't find any past discussion regarding this API design choice, so I'm assuming nobody ever made that suggestion because nobody encountered this issue before. I doubt a change like this, even if it's a breaking change, wouldn't significantly hurt anyone's codebase.